1. Field of the Invention
The present invention relates to an air intake side secondary air supply system for an internal combustion engine.
2. Description of Background Information
In internal combustion engines provided with a three-way catalytic converter in the exhaust system for the purificaiton of exhaust gas, a feedback control is generally executed for controlling the air/fuel ratio in accordance with the composition of exhaust gas and the operating conditions of the engine. This is because an optimum operation of the three-way catalytic converter is attained when the air/fuel ratio of mixture is around a stoichiometric value (14.7:1 for example). An air intake side secondary air supply system for feedback control is an example of the system having air/fuel ratio control operation of this type, in which a secondary air passage leading to a portion of an intake air passage downstream of the throttle valve is provided and the amount of a secondary air flowing therethrough is controlled for performing the air/fuel ratio control.
In the case of internal combustion engines, a combustion state of the engine tends to be unstable during a warm-up period of the engine subsequent to a cold start of the engine. Therefore, in the air intake side secondary air supply system as mentioned above, the supply of the air intake side secondary air is stopped during the warm-up period so that the air/fuel ratio is enriched by stopping the feedback control operation.
For determining the time of restarting of the feedback control of the air/fuel ratio, it is desirable to detect a condition of the restart of the feedback control in response to the temperature of an intake air of the engine, because it is generally recognized that the air/fuel ratio of the mixture to be supplied to the engine varies depending on the temperature of the engine intake air. Therefore, in an example of the conventional system, the supply of the air intake side secondary air is stopped to enrich the air/fuel ratio when the temperature of the engine intake air is equal to or lower than a predetermined temperature t.sub.1 (18.degree. C. for example). Further, in Japanese Utility Model application No. 58-134919 which is assigned to the assignee of the present application, there is proposed an air/fuel ratio control system in which the feedback control of the air/fuel ratio is stopped to enrich the air/fuel ratio if the temperature of a cooling water of the engine is equal to or lower than a second predetermined temperature t.sub.2 and at the same time a vehicle speed is equal to or lower than a predetermined speed V.sub.1 (15 Mile/h for example), even though the intake air temperature is higher than the predetermined temperature t.sub.1. This type of control operation is preferred because the choke valve disposed upstream of the throttle valve is closed to enrich the air/fuel ratio during the cold operation of the engine, and the enrichment of the air/fuel ratio is disturbed by the repetition of the supply and stop of the air intake side secondary air. Moreover, since the proportion of the variation of the secondary air with respect to the amount of the main intake is relatively large when the vehicle speed is low, the feedback control operation may lead to a hunting of the engine rotation which deteriorates the driveability of the engine.
In the air intake side secondary air supply system mentioned above, the feedback control of the air/fuel ratio is executed when the intake air temperature is above the predetermined level t.sub.1 and the vehicle speed is above the predetermined speed V.sub.1 although the cooling water temperature is lower than the predetermined temperature t.sub.2. This is because the vehicle speed is high enough to introduce a relatively large amount of the main intake air under this condition, and the hunting of the engine rotation by the repetition of the supply and stop of the air intake side secondary air is not likely to occur even though the choke valve is not fully opened. Therefore, the purification of the exhaust gas by the feedback control of the air/fuel ratio is given a precedence over the driveability of the engine.
However, since an air intake side secondary air supply of integral control operation or a PI type control operation, which is a combined form of a proportional control operation and the integral control operation, is generally effected, the air/fuel ratio of the mixture to be supplied to the engine will enter into an over-lean range if the engine load is reduced to zero by pressing the clutch pedal to disconnect the power transmission for a deceleration or a gear shift operation when the vehic is running under the above mentioned condition, i.e. when the intake air temperature is above the predetermined level t.sub.1 and the vehicle speed is above the predetermined speed V.sub.1 although the cooling water temperature is lower than the predetermined temperature t.sub.2. Such an over lean air/fuel ratio of the mixture is likely to cause the hunting of the engine rotation which leads to the deterioration of the driveability of the engine.